1. Field of the Invention
The present invention relates to an improved process for preparing methanesulfonic acid from a mixture comprising acetic acid, sulfur dioxide and oxygen by irradiation with light.
Methanesulfonic acid is the simplest representative of the class of alkanesulfonic acids and is of great use for a large number of industrial applications such as the production of metal coatings by electrodeposition or else as esterification catalyst.
2. Description of the Related Art
The most widely used processes for preparing methanesulfonic acid are the oxidation of methyl mercaptan or dimethyl disulfide with oxygen or with chlorine to give methanesulfonyl chloride, followed by hydrolysis. All these processes are associated with problems of toxicity and odor, because the starting materials are formed from hydrogen sulfide, which problems can be overcome only with great technical complexity.
DE 907 053 describes the irradiation of carboxylic acids in the presence of air and sulfur dioxide. The reaction products are the corresponding .beta.-sulfo carboxylic acids.
By contrast, irradiation of acetic acid at room temperature in the presence of air and sulfur dioxide takes a different course, as described in Tetrahedron Lett. 1966, 3095. In the reaction, methanesulfonic acid and also 60%, based on the methanesulfonic acid formed, of sulfuric acid are obtained. However, an industrial process with formation of such a large amount of by-product is uneconomic.